Although digital broadcasting has initially been carried out mainly in satellite broadcasting, a tide of digitization recently rushes into terrestrial broadcasting and cable broadcasting. A waveform equalization technique for reducing transmission line distortion is indispensable for terrestrial digital broadcasting and cable digital broadcasting.
Hereinafter, a conventional waveform equalization apparatus for terrestrial digital broadcasting will be described taking, as an example, a DTV (Digital Television) system using an 8-VSB (8-level Vestigial Side Band) modulation method that is adopted in the United States.
FIG. 11 is a block diagram illustrating a conventional waveform equalization apparatus adapted to the VSB modulation method. In FIG. 11, the waveform equalization apparatus is provided with a data input terminal 1001 to which a DTV signal S1 employing the VSB modulation method (hereinafter referred to as a VSB signal) is applied; a digital filter unit 1015 for performing a filtering process, for waveform equalization on the VSB signal inputted from the data input terminal 1001; a data output terminal 1011 for outputting the VSB signal S2 which has been subjected to the waveform equalization filtering process by the digital filter unit 1015; and a tap coefficient control unit 1014 for calculating tap coefficients to be used in the digital filter unit 1015 on the basis of the data obtained from the digital filter unit 1015, and outputting the tap coefficients to the digital filter unit 1015.
In the digital filter unit 1015, a 32-tap transversal filter (hereinafter referred to as a TF) 1002 receives the signal S1 applied to the data input terminal 1001, and calculates a product of a signal obtained from each of the internal 32 taps and the corresponding tap coefficient which is given by the tap coefficient control unit 1014, and outputs the sum of products so calculated, as a signal obtained by a filtering process for waveform equalization, to an adder 1009 and, further, outputs a delay signal which is obtained by delaying the input signal, to the 32-tap TF 1003 and the adder 1009. The TF 1003 receives the delay signal outputted from the TF 1002, calculates a product of a signal obtained from each of the internal 32 taps and the corresponding tap coefficient which is given by the tap coefficient control unit 1014, and outputs the sum of products so calculated, as a signal obtained by a filtering process for waveform equalization, to the adder 1009. The TF 1002 and the TF 1003 are used as an equivalent to a 64-tap TF, and the delay signal outputted from the TF 1002 to the adder 1009 is regarded as a signal from a center tap of the 64-tap TF constituted by the TF 1002 and the TF 1003, i.e., a main signal component. A delay unit 1004 delays the signal outputted from the adder 1009 by 32 symbols, and outputs the delayed signal to a delay unit 1010 and a slicer 1005. The slicer 1005 maps the output signal from the delay unit 1004 to a closest value among the eight values the VSB signal can take, thereby removing an influence of noise of the input signal on the subsequent processes. A 64-tap TF 1006 receives the signal outputted from the slicer 1005, calculates a product of a signal obtained from each internal tap and the corresponding tap coefficient given by the tap coefficient control unit 1014, and outputs the sum of products so calculated, as a signal obtained by a filtering process for waveform equalization, to the adder 1009, and further, outputs a delay signal which is, obtained by delaying the input signal to a 64-tap TF 1007. The 64-tap TF 1007 receives the signal outputted from the TF 1006, calculates a product of a signal obtained from each internal tap and the corresponding tap coefficient given by the tap coefficient control unit 1014, and outputs the sum of products so calculated, as a signal obtained by a filtering process for waveform equalization, to the adder 1009, and further, outputs a signal which is obtained by delaying the input signal to a 64-tap TF 1008. The 64-tap TF 1008 receives the signal outputted from the TF 1007, calculates a product of a signal obtained from each of the internal 64 taps and the corresponding tap coefficient given by tap coefficient control unit 1014, and outputs the sum of products so calculated, as a signal obtained by a filtering process for waveform equalization, to the adder 1009. The three TFs 1006, 1007, and 1008, which are connected in series, are used instead of a 192-tap TF. The delay unit 1010 delays the signal obtained from the delay unit 1004 by 192 symbols (−64 symbols×3), i.e., by symbols to be delayed by the TF 1006, TF 1007, and TF 1008, and outputs the delayed signal to the tap coefficient control unit 1014. The adder 1009 adds the results of the product-sum operations which are respectively obtained from the TF 1002, TF 1003, TF 1006, TF 1007, and TF 1008 (i.e., signals obtained by the waveform equalization filtering processes of the respective TFs), and outputs a signal obtained by the addition, as a waveform-equalized VSB signal S2, from the data output terminal 1011 and, further, outputs the signal obtained by the addition, to the tap coefficient control unit 1014 and the delay unit 1004. Tho tap coefficient control unit 1014 calculates the tap coefficients corresponding to the respective taps included in the TF 1002, TF 1003, TF 1006, TF 1007, and TF 1008, on the basis of the outputs from the adder 1009 and the delay unit 1010, and outputs the tap coefficients to the TF 1002, TF 1003, TF 1006, TF 1007, and TF 1008, thereby controlling updation of the tap coefficients.
FIG. 12 is a diagram illustrating the structure of a VSB system signal format. The VSB system signal format comprises an area including a data signal 3101 such as video and audio data, an area including a field sync signal 3102, and an area including a segment sync signal 3103.
FIG. 13 is a schematic diagram illustrating the structure of the field sync signal. With reference to FIG. 13, the field sync signal in the area 3102 includes a PN 511 signal 3201, three PN 63 signals 3202, and a control signal 3203. In FIG. 12, a field sync signal #2 is different from a field sync signal #1 only in that the second value of the PN 63 signal 3202 is inverted. Further, in FIG. 13, numeric values on the left side, i.e., +7, +5, +3, +1, −1, −3, −5, and −7, are examples of eight values the 8-VSB modulation method can take.
The VSB signal comprises 832 symbols and 313 segments per frame. Further, the PN 511 signal 3201 is represented by PN511=X9+X7+X6+X4+X3|X|1, and Pre-load is represented by 010000000. The PN 63 signal 3202 is represented by PN 63=X6+X+1, and Pre-load is represented by 100111. The PN 511 signal 3201 comprises 511 symbols, the PN 63 signal 3202 comprises 63 symbols, and the control signal 3203 comprises 128 symbols. Therefore, the whole field sync signal 3102 comprises 828 symbols.
Next, the operation of the waveform equalization apparatus will be described with reference to FIG. 11.
Initially, when an 8-VSB-modulated DTV signal is inputted to the data input terminal 1001 as an input signal S1, the signal S1 is subjected to a waveform equalization process by the TF 1002 and the TF 1003 on the basis of the tap coefficients which are set by the tap coefficient control unit 1014, and the result of product-sum operation performed on the outputs from the internal taps of each of the TF 1002 and TF 1003, as well as a delay signal outputted from the TF 1002, are transmitted to the adder 1009. The adder 1009 sums the inputted signals, and outputs the resultant signal from the data output terminal 1011 as an output signal S2, and further, outputs the signal to the tap coefficient control unit 1014 and the delay unit 1004. The delay unit 1004 delays the output signal S2 by 32 symbols and outputs the delayed signal to the slicer 1005 and the delay unit 1010. The slicer 1005 maps the output of the delay unit 1004 to a closest value among the eight values, thereby removing an influence of noise of the signal on the subsequent processes. Each of the TF 1006, TF 1007, and TF 1008 waveform-equalizes the output of the slicer 1005, and outputs the result to the adder 1009. The delay unit 1010 delays the output of the delay unit 1004 by 192 symbols, i.e., by symbols to be delayed by the TF 1006, TF 1007, and TF 1008, and outputs the delayed signal to the tap coefficient control unit 1014. The delay unit 1010 is provided for outputting the output of the delay unit 1004 which is not octal-leveled by the slicer 1005, to the tap coefficient control unit 1014. The tap coefficient control unit 1014 obtains a difference between the output signal S2 and a most reliable symbol value among the eight symbol values shown in FIG. 13, and performs updation of the tap coefficients of the TF 1002, TF 1003, TF 1006, TF 1007, and TF 1008, using the difference, on the basis of the LMS (Least Mean Square) algorithm.
As described above, the TF 1002 and the TF 1003 function in a similar manner to a single TF, and these TFs serve as a so-called feed-forward type TF. Further, the TF 1006, TF 1007, and TF 1008 also function in a similar manner to a single TF, and these TFs serve as a so-called feed-back type TF, which performs waveform equalization on the signal that is 32 symbols delayed by the delay unit 1004, thereby to realize waveform equalization on the rear region which cannot be waveform-equalized by the TFs 1002 and 1003.
Next, the operation of the tap coefficient control unit 1014 will be described. The tap coefficient control unit 1014 obtains a difference between the inputted output signal S2 and the most reliable symbol value among the eight symbol values, and performs updation of the tap coefficients of the TF 1002, TF 1003, TF 1006, TF 1007, and TF 1008 on the basis of the LMS algorithm. The LMS algorithm is an algorithm for performing the n-th updation (n: positive integer) of a tap coefficient Ci of a tap i (i: positive integer) in any of the TF 1002, TF 1003, TF 1006, TF 1007, and TF 1008 on the basis of the following formula (1).Ci(n+1)=Ci(n)−α×en×di  (1)where α indicates a fixed constant step size for deciding a coefficient updation amount, en is an error in the output signal, di indicates data of the tap i, and −α×en×di indicates the coefficient updation amount. The tap coefficient control unit 1014 updates the tap coefficients to be used in the TF 1002, TF 1003, TF 1006, TF 1007, and TF 1008 on the basis of formula (1). The output signal S2 is represented by the following formula (2).S2(n)−Σ(i=0, 255)(Ci(n)×di)  (2)
Next, a description will be given of a conventional waveform equalization apparatus used for cable digital broadcasting. Also in cable digital broadcasting, waveform equalization using the LMS algorithm is carried out. However, cable digital broadcasting usually employs, as a modulation method, not the 8-VSB modulation method but a QAM (Quadrature Amplitude Modulation) method.
FIG. 14 shows an arrangement of signal points, with respect to a DTV signal using a 64-state QAM method (hereinafter referred to as a 64-QAM signal). In FIG. 14, the abscissa shows the real number axis while the ordinate shows the imaginary number axis.
FIG. 15 is a block diagram illustrating a waveform equalization apparatus adapted to the 64-state QAM method. With reference to FIG. 15, a real component of a 64-QAM signal is inputted to a data input terminal 2001. The inputted real component of the 64-QAM signal is inputted to a TF 2003 and a TF 2005. The TF 2003 performs a product-sum operation of the outputs from the respective internal taps, and outputs the result as a signal obtained by a filtering process for waveform equalization, to a subtracter 2007. Further, a main signal of the TF 2003 is inputted to an adder 2009. Further, the outputs from the respective taps of the TF 2003 are inputted to a tap coefficient control unit 2013. The TF 2005 performs a product-sum operation of the outputs from the respective internal taps, and outputs the result to an adder 2008 as a signal obtained by a filtering process for waveform equalization.
On the other hand, an imaginary component of the 64-QAM signal is inputted to a data input terminal 2002. Then, the imaginary component of the 64-QAM signal is inputted to a TF 2004 and a TF 2006. The TF 2006 performs a product-sum operation of the outputs from the respective internal taps, and outputs the results to the adder 2008 as a signal obtained by a filtering process for waveform equalization. Further, a main signal of the TF 2006 is inputted to an adder 2010. Furthermore, the outputs from the respective taps of the TF 2006 are inputted to the tap coefficient control unit 2013. The TF 2004 performs a product-sum operation of the outputs from the respective internal taps, and outputs the result to the subtracter 2007 as a signal obtained by a filtering process for waveform equalization.
The subtracter 2007 subtracts the output of the product-sum operation result of the TF 2004 from the output of the product-sum operation result of the TF 2003. The adder 2009 adds the output of the subtracter 2007 and the main signal of the TF 2003, and outputs the sum as a real component of a complex output signal S22 from the data output terminal 2001 and, further, inputs the sum to the tap coefficient control unit 2013.
The adder 2008 adds the output of the product-sum operation result of the TF 2006 and the output of the product-sum operation result of the TF 2005. The adder 2010 adds the output of the adder 2008 and the main signal of the TF 2006, and outputs the sum as an imaginary component of the complex output signal S22 from the data output terminal 2012 and, further, inputs the sum to the tap coefficient control unit 2013. The tap coefficient control unit 2013 controls updation of the tap coefficients of the TFs 2003˜2006, on the basis of the real component and imaginary component of the complex output signal S22.
The complex output signal S22 obtained from the waveform equalization apparatus is represented by the following formula (3).S22(n)=Σ(i=0, 255)(Ci(n)×di)  (3)
In formula (3), when Ci(n) and di are represented by complex elements as follows:Di−di(r)+jdi(i)Ci(n)=Ci(n)(r)+jCi(n)  (i)where (r) indicates real component data, and (i) indicates imaginary component data, the complex output signal S22 is represented as follows:S22(n)=Σ(i=0, 255)((Ci(n)(r)+jCi(n)(i))×(di(r)+jdi(i))) =Σ(i=0, 255)((Ci(n)(r)×di(r)−Ci(n)(i)×di(i))+j(Ci(n)(r)×di(i)+Ci(n)(i)×di(r)))
When S22(n) is represented byS22(n)=S22(n)(r)+jS22(n)(i)S22(n)(r) and JS22(n)(i) are represented by the following formulae (4) and (5), respectively.S22(n)(r)=Σ(i−0, 255)(Ci(n)(r)×di(r)−Ci(n)(i)×di(i))  (4)S22(n)(i)=Σ(i=0, 255)(Ci(n)(r)×di(i)+Ci(n)(i)×di(r))  (5)
Next, the tap coefficient updating operation by the tap coefficient control unit 2013 will be described.
The tap coefficient control unit 2013 controls updation of the tap coefficients of the TFs 2003˜2006 on the basis of the real component and imaginary component of the inputted complex output signal S22 as well as the tap outputs from the TF 2003 and the TF 2006. A tap coefficient updation formula to be used in the case where a QAM signal is inputted is represented as follows, using the LMS algorithm, in a similar manner to the above-mentioned formula (1).Ci(n+1)−Ci(n)−α×en×di*  (6)where di* indicates the complex conjugate (complex number) of di.
When formula (6) is developed using complex expressions as follows:Ci(n)−Ci(n)(r)+jCi(n)(i)en=en(r)+jen(i)di=di(r)+jdi(i) di*=di(r)−jdi(i)the real component is represented byCi(n+1)(r)=Ci(n)(r)−α×{en(r)×di(r)+en(i)×di(i)}and the imaginary component is represented byCi(n+1)(i)=Ci(n)(i)−α×{en(i)×di(r)+en(r)×di(i)}
The conventional waveform equalization apparatuses are constituted as described above, and a DTV signal with reduced distortion can be obtained by employing these waveform equalization apparatuses.
In the conventional waveform equalization apparatuses, however, the calculation of the output signal varies between the case where the VSB signal is inputted and the case where the QAM signal is inputted, as shown by formula (2) for the VSB signal and formulae (4) and (5) for the QAM signal, and therefore, separated waveform equalizers adapted to the respective signals must be prepared. Accordingly, when providing a waveform equalization apparatus adaptable to both of the VSB signal and the QAM signal, waveform equalizers adapted to the respective signals are required and these equalizers should be switched according to the input signal, resulting in an increase in the circuit scale.
Furthermore, the tap coefficient control unit 1014 calculates the tap coefficients using an error in the VSB signal, according to the LMS algorithm as shown by formula (1). Therefore, the signal before being sliced by the slicer 1005 must be delayed by a time equal to the time to be delayed by the TF 1006, TF 1007, and TF 1008, before it is inputted to the tap coefficient control unit 1014, and the delay unit 1010 is required as means for delaying the signal. However, such delay unit is large in circuit scale, resulting in an increase in the circuit scale of the whole waveform equalization apparatus.
Furthermore, in the conventional waveform equalization apparatus, adaptive control for optimizing the filter characteristic is carried out by updating the tap coefficients with the tap coefficient control unit 1014 on the basis of the output signal. Therefore, the filter characteristic is not constant but varies with the updation of the tap coefficients. As a result, it is difficult for the user to know the filter characteristic that is updated by the adaptive control.